1. Field of the Invention
The present disclosure relates to energy storage systems and more particularly to the mechanical and heat storage of energy at an earlier time for use at a later time.
2. Description of the Related Art
Electricity generating devices such as wind turbines, photovoltaic panels and hydroelectric power systems generate electricity without burning fossil fuels. These generators use renewable sources such as wind, sun and falling water to power the generators. In order to react to potential interruptions in renewable sources and to react to changing grid load demands, electricity storage is necessary.
Presently, batteries are the storage choice in residential and commercial buildings. Low cost batteries typically have round trip efficiencies (RTE) between 70 to 85% and cost 200-600 $/kWh (Li-ion batteries are 85-95% efficient and cost 600-1200 $/kWh.) Unfortunately, batteries have a number of drawbacks. Unless they are charged from a DC source, a AC-DC converter results in additional cost and efficiency losses. Repeated cycling degrades the storage capacity of batteries and battery life is also shortened when batteries are allowed to fully-discharge. Large scale deployment is also limited in high rise buildings due to fire risks.
Pumped storage hydroelectricity (PSH) stores water in an elevated reservoir when electricity demand is low and releases the water under the force of gravity to generate electricity when electricity demand is high. PSH is a mature technology with relatively high RTE of between 75-80%. However, PSH has poor expansion prospects in the US because site selection and approval is often difficult, and most favorable sites are already exploited.
Where PSH is not an option, compressed air energy storage (CAES) is often used.
However, because of large compressor losses, CAES has low RTE of about 40-55% for conventional diabatic CAES. With advanced CAES technologies such as adiabatic CAES and isothermal CAES, RTEs up to and above 70% are achievable.
Rail energy storage uses heavily-loaded rail cars and electric motors/generators to store energy on long, low-angle earthen inclines. When energy is abundant, the rail cars ascend the incline using an electric motor powered by a renewable source or the grid. When energy demand is high, the rail cars descend the incline and the motor functions in reverse as a generator to produce electricity. Very long slopes with 6-8% grade are needed, which limits the geographical areas where rail energy storage systems can be built.
Increasing the penetration of renewable electricity generation requires low cost, highly round trip-efficient energy storage systems to smooth intermittent power production and store the power for use when demand is high. There is a great need for high-efficiency, highly scalable, cost-effective storage systems that are environmentally benign, flexible in operation, and that do not require large physical footprints.